Diodes allow current to flow in one direction. However, if a high enough reverse voltage is applied to a diode, the diode will allow current to flow in the opposite direction through the diode. The voltage required to drive current though the diode in the reverse direction is referred to as the reverse breakdown voltage, or simply the breakdown voltage. Some diodes, such as Zener diodes, have relatively low breakdown voltages that are utilized in various circuit designs. For example, electrostatic discharge (ESD) protection circuits may utilize Zener diodes to block current from flowing under normal voltage conditions, and to allow current to flow through the diode in the reverse direction under high voltage conditions.